1. Field of the Invention
The present invention relates to a pressure detector utilizing a piezo effect of a semiconductor and in particular to a pressure detector which may be used for detecting combustion gas of an internal combustion engine.
2. Description of the Related Art
Japanese Patent Laid-Open No. 64-36081 discloses one such pressure detector.
Pairs of output and input electrodes are provided to face each other such that they cross at a right angle on a single crystal silicon body having a homogeneous impurity concentration and a thickness cut out so as to have a crystal face of (110) as a face to which a compression force is applied. Then, a seat which can always disperse and transmit the compression force applied perpendicularly to the crystal face is bonded to the crystal face. Further, a support base having sufficient rigidity in the direction where the compression force is generated is bonded to another face of the single crystal silicon body facing to the crystal face. The single crystal silicon body is thus caused to generate only a simple compression force when the compression force acts perpendicularly on the crystal face by bonding it to the support base.
By forming the pressures in this manner, the influence of changes in the resistance value of the strain gauge, which increases with a change in temperature, is is reduced.
Here the single crystal silicon body is constructed as indicated by the reference numeral 62 shown in FIG. 24 and a .pi.'.sub.63 gauge is used which generates a potential between output electrodes 64a and 64b disposed in the y-direction which cross with input electrodes 63a and 63b when a current is applied in the x-direction from the input electrodes 63a and 63b and a compressive force is applied in the z-direction. The reference numeral 65 denotes a pressurized surface where the seat pressurizes the crystal face.
It is to be noted that because normally a planar technology by means of patterning using a mask is implemented when the electrodes are formed, a certain degree of electrode width W is formed as shown in FIG. 24. Further, because the electrodes are formed from a metal such as Al, the resistance within the electrode is very small.
Another pressure detector of this sort shown in FIG. 25 is disclosed in Nippondenso Technical Disclosure No. 92-220.
In FIG. 25, a cup 69 having a metal diaphragm portion 68 is provided at an opening on the side of distal end of a housing (case) 67 having a screw section 66. Provided within it is a support base 71 for holding leads by hermetically sealing them. A pressure sensitive element 72 on which a gauge resistor (not shown) is formed is provided on the support base 71 and a rod 73 having a globular or circular top face for transmitting a load is provided further on the element 72. A heat radiating agent 74 is provided under the support base 71. Because the apparatus is connected to an engine block (not shown) by the screw section 66, the support base 71 electrically conducts with the engine block and becomes a ground.
A gap between the rod 73 and the pressure sensitive element 72 and that between the pressure sensitive element 72 and the support base 71 are bonded by an insulating adhesive 75. Each gap is thus insulated by an insulating adhesive layer made from the adhesive 75.
A gap between the pressure sensitive element 72 and the lead 70 is electrically connected by a bonding wire 76.
According to the pressure detector constructed as described above, pressure applied to the diaphragm portion 68 is led to the pressure sensitive element 72 via the rod 73 and is detected by a piezoresistive effect caused in the gauge resistor on the pressure sensitive element 72.
According to the pressure detector disclosed in Japanese Patent Laid-open No. 64-36081 described above, however, a dislocation of the seat is brought about more or less when it is bonded to the crystal face in reality and there is a problem that the coordinate of the position collapses and the sensitivity drops at that time since the gauge resistor is disposed on the whole surface of the single crystal silicon body 62 and the pressurized surface 65 on the surface, i.e. the surface on which the compression force acts, is pressed by the seat in the prior art structure.
There is also another problem that since the gauge area is larger than the pressurized surface 65, a temperature difference is brought about between the pressurized surface 65 right below the seat and other areas than that by heat from the seat when used in an internal combustion engine and that the temperature characteristics vary, especially when the seat is dislocated from the pressurized surface 65 as described above.
Furthermore, because the single crystal silicon body 62 is constructed to detect a potential by one gauge resistor, the resistance within the electrode becomes very small and the potentials become almost equal in the electrode having the certain degree of electrode width W and made from a metal as described above when the compression force is applied to the crystal face. Accordingly, there is a problem that the potentials specially generated are canceled out, thereby degrading the sensitivity.
Next, in the pressure detector disclosed in the disclosure No. 92-220 described above, because there exists a small irregularity on the surface of the support base 71 as shown in FIG. 26 which is a partially enlarged view of circled portion G in FIG. 25, the support base 71 and the pressure sensitive element 72 may partially contact and conduct. Accordingly, they cannot be sufficiently insulated just by the insulating adhesive layer. In this case, there is a problem that a leak current flows to the case 67 between the case and the gauge resistor due to the non-insulation between the pressure sensitive element 72 and the support base 71, thereby causing unstable characteristics.
Normally, the gauge resistance is of the order of several hundreds ohms to several tens kiloohms and when the gauge resistance is capacity-coupled with the case 67 with a capacity of several tens pF, for example, a high frequency noise of more than several tens MHz is mixed into the gauge output signal, causing erroneous operation of the apparatus. Through the study on the fact that the high frequency noise is mixed into the gauge output signal by the parasitic capacitance, it was found that the parasitic capacitance is generated at three places: the hermetic sealed section of the lead 70, the insulating adhesive layer between the support base 71 and the pressure sensitive element 72 and the insulating adhesive layer between the pressure sensitive element 72 and the rod 73. Further, it was found that the high frequency noise is mixed into the gauge output signal by the capacitance coupling of each, causing erroneous operation of the apparatus.
Accordingly, the prior art structure has the problems in that the high frequency noise mixes into the gauge output signal by the parasitic capacitance at those three places, causing erroneous operation of the apparatus.
As described above, characteristics of these pressure detectors fluctuate, with temperature characteristics due to the dislocation of the seat and the electrode width W. Further, current is prone to leak from the non-insulation between the pressure sensitive element and the support base, and high frequency noise is generated and mixed into the gauge output signal due to the parasitic capacitance generated at the hermetically sealed portion of the lead, in the insulating adhesive layer between the support base and the pressure sensitive element and in the insulating adhesive layer between the pressure sensitive element and the rod.